The　three-dimensional　thermal　characteristics　of　micro-light-emitting　diodes　(mu　LEDs)　on　GaN　and　sapphire　substrates　were　studied　with　forward-voltage　methods,　thermal　transient　measurements,　and　infrared　thermal　imaging.　The　mu　LEDs　on　the　GaN　substrate　showed　an　approximately　10　degrees　C　lower　junction　temperature　and　smaller　amplitude　of　the　K　factors　than　those　on　the　sapphire　substrate　under　the　current　injection　level　of　4　kA　cm(-2).　The　thermal　transient　measurement　showed　that　the　spreading　thermal　resistances　of　the　mesa,　the　GaN　epilayer,　and　the　interface　of　the　GaN/substrate　were　reduced　significantly　for　mu　LEDs　on　the　GaN　substrate　because　of　the　high-quality　GaN　crystal　and　the　homogeneous　interfaces.　The　infrared　thermal　images　showed　lower　total　average　junction　temperatures　and　more　uniform　temperature　distributions　for　the　mu　LEDs　on　the　GaN　substrate,　which　were　also　simulated　with　APSYS　software.　The　thermal　transport　mechanisms　are　discussed　for　the　lateral　and　vertical　directions　in　the　mu　LEDs.